Magnetic hard disk drives comprise the primary means of storing programs and data for modern computer systems, ranging from very high performance "main frame" computer systems, to small "notebook PC's". The disk drives used are always designed to comprise the best compromise between size, performance, capacity and cost for the specific application.
In notebook PC's, the need for a small size is apparent to simply allow all of the components of the computer to fit within the small physical size of the computer. Small size for given performance and capacity has become very important for all other computer systems also, due to the need to conserve the floor space occupied by the computer system.
The rotary actuator disk drive has become the dominant type of magnetic hard disk drive in the industry because of its smaller size for the same capacity, as compared to linear actuator hard disk drives.
Design improvements are constantly being made to rotary actuator disk drives to increase their data capacity without dramatic increases in cost. Examples include the continuing increase in track density and in linear data density by use of ever smaller magnetic heads, such as the change from thin film inductive heads to inductive write and magneto-resistive read heads.
Improvements in track density and linear density provide significant improvements in data capacity for a given disk, but do not provide any improvement in performance, specifically in the ability to access the data on the disk. In fact, the increase in capacity of each disk in a disk drive makes it more difficult to access the data, since the tracks are now closer together and the smaller geometry of the head requires that the servo system be designed to operate the same actuator more precisely than previously. An actual loss in performance comes from the increased "settle time" for the actuator to come to rest over the desired track at the end of a seek. Settle time is not well defined in the art, but essentially includes the time required to bring the head to a stationary position from the seek, including any backwards movement after overshoot.
In the past, attempts have been made to increase the performance of disk drives by adding an additional actuator. The additional actuator has been a duplicate of the first and would access all of the disks of the drive to allow the data to be accessed more quickly by having one actuator seek to new data while the head on the other actuator was reading or writing the previous data. Some examples of identical dual actuators include U.S. Pat. No. 5,341,351, Ng, assigned to the same assignee as the present invention; U.S. Pat. No. 5,355,486, Cornaby, assigned to Conner Peripherals, Inc.; and U.S. Pat. No. 4,270,154, Crawford, unassigned. Japanese patent publication no. 56-130865, Nagai, assigned to Hitachi Seisakusho K.K., appears to illustrate non-identical dual actuators, but does not discuss any difference. Such dual actuators, however, dramatically increase both the cost of the drive and the space requirement for the drive.
The increase in performance by providing dual or duplicated actuators is helpful. However, such dual actuator drives are not being implemented in modern disk drives with any amount of success.
In an attempt to reduce both the cost of the drive and the space requirements, the dual actuators can be mounted on the same pivot axis. One example is U.S. Pat. No. 4,544,972, Kogure et al., assigned to Nippon Telegraph & Telephone Public Corporation. The actuators thus access only separate disk surfaces of the drive but allow use of the same pivot axis and the same back iron for the voice coil motors of the two rotary actuators.
Such an arrangement greatly increases performance with only a small increase in cost. However, such a design still faces an additional challenge posed by modern applications. The conflict is between increasing capacity required by such things as image and video data versus the small record "character" or "text" data, which is best served by a higher performance disk drive.
The dramatically increasing requirements for capacity relates to the increase in image and video data in modern computer systems, as discussed above. Such data has very different characteristics from the programs and the "character" data primarily used in computers of the past. Whereas the programs and "character" data sets of the past utilized short data records, the image and video data are characterized by very large data sets. Such large data sets do not require the constant accessing and performance that the shorter data sets and programs require. The '972 patent, having identical actuators, cannot resolve these conflicting requirements.
Thus, what is needed is a disk drive with high capacity and with the capability of increasing performance appropriate to the data.